This invention relates to nuclear reactors and more particularly to an emergency core coolant system for cooling the core of a nuclear reactor.
In the event of an accident in which there is a break in the reactor coolant system, it has been postualted that the entire coolant medium which absorbs and removes the heat generated in the nuclear core will be lost or at least considerably decreased. Although control elements are inserted into the core to terminate the fission process upon the occurrence of such an accident, decay heat generated by the already formed fission products is capable of causing fuel or clad melting if sufficient cooling is not supplied to the fuel. Furthermore, the overheating of the fuel cladding can result in a severe adverse chemical reaction with its environment, which may not be reversed by later cooling procedures. Accordingly, it is necessary to provide a sufficient coolant flow immediately to insure that this heating of fuel and cladding does not occur.
In some prior art systems, emergency core coolant water is normally injected into the inlet nozzles of the reactor and allowed to flow downwardly along the normal coolant path to the bottom of the vessel, and then upwardly to the nuclear core. Obviously, with such a system there is necessarily a time delay since the emergency coolant must first flood the bottom of the reactor before flowing upward into the core. Furthermore, the coolant injected into the reactor generates large quantities of steam which may create a pressure buildup in the core and outlet plenum of the reactor, thereby impeding further coolant from reaching the core.
In other prior art systems a special plenum for distributing the emergency coolant fluid is provided in a active region of the reactor. This plenum is fluid coupled to an external source of fluid and means are provided for discharging the coolant supplied to the plenum into the affected regions of the reactor. In these systems the inactive region is normally located above the outlet flow region of the reactor with the special plenum being defined by the reactor vessel and the upper plate of the guide structure or by a separate header component positioned above the guide structure.
It is to an improved form of the special plenum type of emergency core cooling system that the present invention is directed.